Having bones on the outside of limbs is a good idea. Engineers, for example, use tubes in all kinds of different ways. The optimal shape for a tube is influenced by numerous factors, but generally speaking, it is better to have a relatively large diameter and a thin wall. This provides resistance to bending and torsion.
Professor David Taylor
Researchers from Trinity College Dublin have discovered that – in terms of strength – the skeletal forms of insects and crabs are superior to those of human beings. The team, whose findings were published in the Journal of the Royal Society Interface
, found that if human leg bones were formed from cuticle – the material that comprises arthropod exoskeletons – they could be twice as strong as they are in actuality.
The interdisciplinary study, which involved elements of engineering mechanics, materials science and biomechanics, found that the legs of grasshoppers and crabs have the ideal shape to resist bending and compression. As cuticle is amongst the toughest of natural materials, having bones on the outside of their bodies helps to protect such animals.
The scientists set out to compare human leg bones, which take the form of thin, solid rods, with those of arthropods, which consist of hollow tubes. Whilst rods tend to bend easily, hollow tubes are prone to buckling. X-ray images generated by a special computer-tomography machine allowed the team to examine insect legs at a resolution of only a few thousandths of a millimetre. In subsequent comparisons, the researchers discovered that in relation to their radii, the legs of insects and crabs have much thinner walls than those of humans. Crabs were found to have the ideal composition to resist both bending and buckling.
In an interview with ScienceOmega.com
, Professor David Taylor from the Trinity Centre for Bioengineering explained how – from a design engineer’s perspective – the human thighbone might benefit from a redesign.
"I would like to significantly alter its shape," he replied. "I would change the diameter and thickness of the bone making it correspondingly thinner. We calculated that this would deliver twice the strength for the same weight of bone. Looking at it another way, you could save weight whilst maintaining bone strength. Of course, I’m not recommending this as a new form of plastic surgery.
"Essentially, I am talking about the shape of a crab’s leg. In terms of strength, exoskeletons make sense. Having bones on the outside of limbs is a good idea. Engineers, for example, use tubes in all kinds of different ways. The optimal shape for a tube is influenced by numerous factors, but generally speaking, it is better to have a relatively large diameter and a thin wall. This provides resistance to bending and torsion."
Arthropods, such as arachnids, insects and crustaceans, have existed within the vast majority of ecosystems for millions of years. I asked Professor Taylor whether strength is the main reason behind the exoskeleton’s evolutionary success.
"Obviously, there are an awful lot of different factors to consider," he answered. "Strength is only one of these. Even so, it certainly has allowed arthropods to optimise the shape of their bones. Mammals, on the other hand, have been unable to do the same with their endoskeletons."
Whilst the strength of arthropods’ skeletal structures might be superior to that of their mammalian counterparts, Professor Taylor pointed out that we still have some tricks up our collective sleeve.
"Our bones have the ability to repair themselves," he noted. "As you know, if you break your arm, it will heal. What you might not realise is that your bones are continually
repairing themselves. As you go about your daily business, you are putting stress on your bones and they are gradually becoming damaged. However, your bones are full of living cells and these cells are capable of detecting and repairing this damage.
"As far as we know, insects and crustaceans don’t have this ability. I must admit that this issue hasn’t yet been fully investigated. It’s an area that we intend to examine during our next research project, although if I were to hazard a guess, I’d predict that exoskeletons won’t be able to repair themselves as effectively as endoskeletons."
My final question to Professor Taylor concerned the potential practical applications of his research. Might his findings be used within fields such as robotics and bioengineering?
"It is always a good idea to look to nature for potential solutions to problems," he replied. "This is the philosophy that underlies biomimetics. Of course, engineers already know a great deal about tubes and how best to design them. There is, however, scope to look at their composition. In nature, you tend to find variation within the composition of individual tubes. Bamboo, for example, is not made from just one substance. It is what we call a graded structure. This is an area that has not yet been fully exploited by scientists and engineers."